O1 Tempering Issues

L

Lynn Westfall

Joined
Aug 6, 2007
Messages
58
I mentioned my issues in another thread but thought it best to start another.

I've been a hobby knife maker for over 10 years. I finally got around to building my salt pot earlier this year and have been experimenting with it as time allows. Over the last couple of weekends, I've been working on a batch of O1 knives that is giving me trouble. My heat treating procedure is:

1500 F - 20 minute soak
Quench into 400 F Salt - hold aprox 5 minutes
Cool to room temperature and then into Liquid Nitrogen for 4 hours +

1st Temper 450 F (note kitchen oven checked with two thermometers)

The problem knife is approximately 1/8" thick , 1" wide blade, flat ground to an edge thickness of approximately 20/1000. I put a 13 degree bevel using my EdgePro.

2nd Temper 475 F - 1.5 hours

Brass Rod Test using a 1/4" Brass Rod - result chipping

3rd Temper 500F - 1.5 hours - result chipping, (less than before, but unacceptable)

I've seen various manufactures data showing tempering ranges from 450F to 500F to achieve 59-60Rc. I've never seen data recommending tempering over 500F.

(Note: I also did up three 1/16" thich small utility blades. The edges all flexed wonderfully after a 475 F temper.)

My Questions:

How should I proceed from where I am at? Temper longer? Higher Temper?
(I'm worried about giving up too much edge holding if I go much higher.)

Is a 13 degree per side bevel too thin? I would probably put a 15 degree microbevel on the knife before giving it to the end user but feel that it should be able to handle that thin an edge. I believe I am performing the brass rod test correctly but am open to suggestions.

My next step will be to boost the temperature up to about 515F for two hours and then comparison test the edge holding ability. I have rope that I use. Rope cutting performance has increased greatly since getting the salt pot set up. (My wife is none too happy about her kitchen over running hot for a couple of hours in this Texas heat.)

All input would be appreciated.
Thanks,
 
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I have no answers for you, but I do question a 400° quench. I know very little of salt pot heat treating. Is this a common quench temp for 01 when using salts? Kevin? Mete'?
 
It's the basic marquenching .You quench it into 400 F oil or better salt which for this steel is the Ms temperature .Then it slowly cools in air minimizing the quenching stresses.
I don't know offhand what the problem is though I wonder how valid the rod test is.
 
Mete, you are correct in questioning the validity of my brass rod test. It is somewhat subjective. However, it does give me a good indication that this knife would probably chip out if pushed hard against bone.

This particular blade would see use in a general purpose hunting environment. As always, there is a trade off between edge holding ability and toughness/ability to resist chipping.

I'm going to see if I can find a place in San Antonio where I can have it Rockwell tested. My fear in going with too high a temper is that I will be giving up too much in edge holding.

This is the first time that I have hardened O1 at 1500F. In the past I have used 1480F. Perhaps my edge angle is too thin for this steel and application. I'll have to take a look at all of my variables and decide how to proceed in the future.
 
First off I think that 20 min soak for 1/8 in stock is too long.
Second why quench in 400 deg salt? I don't think that you are getting the temp down fast enough.
Third why not just temper at 400deg in your salt bath for 2 one hour cycles?

I don't think that the nitrogen soak is needed for O-1, that might be your problem.
 
When I do O1 I do about a 10-15 min soak at 1450F and then quench in 125F light oil followed by 2 tempers at 400f 2 hours each. I do not let the blade cool below 400f before the first temper. I have one of 8 blades on the bench right now, I plan on testing one including bending and general distruction, I have not had any edge problems in the past.
 
I need to look at the source of my heat treat instructions but I had read not to let the blade fully cool prior to the first temper. I use a digital thermometer to check the blade during the quench, I have the oven hot and when time comes I pull from the quench, quick wipe of the oil and into the oven at 400 for two hours.

Seems lynn is doing the same thing with his 400f salt bath unless he lets it cool prior to tempering.

Do you see something wrong there?
 
I appreciate the responses to my new dilema. I'm still hopeful that I'm getting my O1 as hard as possible and just need to temper a little higher and longer to acheive my desired result and still have good edge holding.

A clarification to my heat treating procedure. I quench into 400F salts and hold for about 5 minutes. I then let the blades cool all the way down to room temperature before going into cryo.

Someone asked why I don't temper in my low temp salts which is a good question. Unlike my high temp salt pot, the low temp is manually controlled. I can hold temp easy enough during quenching but couldn't for two hours without having to constantly watch it. Getting a PID and gas regulator set-up for the low temp salt is on the agenda.
 
Hardening:
Pre-heat slowly to 1200ºF - 1250ºF, then soak steel thoroughly. Increase heat more rapidly to quenching temperature of 1450ºF-1500ºF.
Hold steel at quenching temperature for one half hour per inch of greatest cross section.
Surface protection: pack hardening or controlled atmosphere furnaces.
Quench in oil-bath (Oil temperature should be 150ºF).


Quenching Temperature, °F Fracture Grain Size Rockwell C
1400 9 60
1425 9 62
1450 9-1/2 63
1475 9-1/2 65
1500 9-1/2 65
1525 9-1/4 65
1550 9-1/4 65
 
Patrick, I am attaching a TTT for O-1. You'll notice at 400F it says "Beginning of martensite", or what we usually call "Ms" for "martensite start".
Below that you'll notice some numbers 40%...95%...99%, the amount of martensite formed by a certain temp. At 99%, this is nominally called "martensite finish", Mf. In this case, it is around 150F. This temp should be reached prior to any tempering, or conversion will be inhibited and undesirable things can happen.

In Lynn's case, holding for a bit at 400F (Ms) allows the martensite to start forming without unduly stressing out the blade. (Too long at 400F is not a good thing, either, if the time crosses that curve at the right.) It helps avoid fractures (both macro- and micro-) and warping, among other things. After that "marquenching" it still needs to cool to the Mf before tempering.

Hope that helps, and hope I said it all properly. Corrections welcome!.
 
I believe 20 minutes is too short a soak time to get all of the goodies in the O-1 into play. you will not damage the steel if you soak it longer than necessary as long as your temp control is good

-Page
 
How are you doing the brass rod test? Is it as described in Wayne Goddards book? If so, chipping (or curling) would be very abnormal, and suggest that something is seriously wrong with that piece of steel... probably more to it than just making minor heat treating adjustments if it's still chipping. 01 should be plenty tough at higher hardnesses.
 
Can someone please explain to me how to read Fitzo's chart. I feel kinda stupid asking. But I've printed it and stared at it for a while. Still, I can't seem to get anything out of it.

Sorry for being drole.
 
The graph is a TimeTemperatureTransformation diagram. It tells you what microstructures get as you cool down from austenitizing temperature. Each steel has it's own graph.If you cool slowly you will get pearlite .A bit faster you get bainite and fast cool will give you martensite. In each case there are two lines ,one for the start of transformation and one for the completion. For proper hardening of O-1 you have to cool quickly enough to avoid the 'pearlite nose' .This will give you a fully martensitic structure.
Going through the martensite transformation fast will increase the transformation stresses and the chance for cracking.Therefore you can cool quickly to the Ms temperature then cool slowly [air cool] to minimize stresses. This is called marquenching.
 
Here is a Kevin C. thread which discusses reading that TTT chart and its implications, Andy. Hope it helps.

Edited to add: That thread I link goes well beyond just reading the TTT chart and would be an excellent read for any maker. For that matter, anyone with search capabilites would do well to search "All threads started by" Kevin and sift through the topics. There are a many gems of HT info in there.
 
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mete said:
The graph is a TimeTemperatureTransformation diagram. It tells you what microstructures get as you cool down from austenitizing temperature. Each steel has it's own graph.If you cool slowly you will get pearlite .A bit faster you get bainite and fast cool will give you martensite. In each case there are two lines ,one for the start of transformation and one for the completion. For proper hardening of O-1 you have to cool quickly enough to avoid the 'pearlite nose' .This will give you a fully martensitic structure.
Going through the martensite transformation fast will increase the transformation stresses and the chance for cracking.Therefore you can cool quickly to the Ms temperature then cool slowly [air cool] to minimize stresses. This is called marquenching.
Click to expand...

So once the steel reaches MS (400) it is not important how quickly it gets down to MF (150)???
 
If I'm reading Fitzo's graph correctly, you wouldn't want to leave it at or near 400 for longer than 10 minutes or you can start crossing the pearlite line.

Erin
Graph-Whisperer

Fiddleback said:
So once the steel reaches MS (400) it is not important how quickly it gets down to MF (150)???
Click to expand...
 
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